requirements: Add back explicit typing dependencies.

These are needed to make the output the same on Trusty and Xenial.

requirements/docs.in

@@ -24,3 +24,6 @@ CommonMark==0.5.4
 docutils==0.14
 imagesize==0.7.1
 snowballstemmer==1.2.1
+
+# Include typing explicitly, since it's needed on Python 3.4
+typing==3.6.2

requirements/docs.txt

@@ -26,5 +26,5 @@ snowballstemmer==1.2.1
 sphinx-rtd-theme==0.2.4
 sphinx==1.6.5
 sphinxcontrib-websupport==1.0.1  # via sphinx
-typing==3.6.2             # via sphinx
+typing==3.6.2
 urllib3==1.22             # via requests

requirements/mypy.in

@@ -3,3 +3,6 @@
 # and requirements/mypy.txt.
 # See requirements/README.md for more detail.
 mypy==0.540
+
+# Include typing explicitly, since it's needed on Python 3.4
+typing==3.6.2

requirements/mypy.txt

@@ -9,4 +9,4 @@
 #
 mypy==0.540
 typed-ast==1.1.0          # via mypy
-typing==3.6.2             # via mypy
+typing==3.6.2